1. Field of the Invention
The present invention relates to a recombinant non-pathogenic infectious bursal disease virus (IBDV) and methods of delivering via in ova for protection against virulent challenge by classic and variant strains of IBDV.
2. Description of the Related Art
Infectious bursal disease virus (IBDV), a member of the Birnaviridae family, is the causative agent of a highly immunosuppressive disease in young chickens (Kibenge, et al., 1988). Infectious bursal disease (IBD) or Gumboro disease is characterized by the destruction of lymphoid follicles in the bursa of Fabricius. In a fully susceptible chicken flock of 3-6 weeks of age the clinical disease causes severe immunosuppression, and is responsible for losses due to impaired growth, decreased feed efficiency, and death. Susceptible chickens less than 3 weeks old do not exhibit outward clinical signs of the disease but have a marked infection characterized by gross lesions of the bursa.
IBDV is a pathogen of major economic importance to the nation and world's poultry industries. It causes severe immunodeficiency in young chickens by destruction of precursors of antibody-production B cells in the bursa of Fabricius. Immunosuppression causes increased susceptibility to other diseases, and interferes with effective vaccination against Newcastle disease, Marek's disease and infectious bronchitis disease viruses.
The capsid of the IBDV virion consists of several structural proteins. As many as nine structural proteins have been reported but there is evidence that some of these may have a precursor-product relationship. The designation and molecular weights of the viral proteins (VP) are as shown below.
Viral ProteinMolecular WeightVP190 kDaVP241 kDaVP332 kDaVP428 kDaVP5 (NS)17 kDa
The IBDV genome consists of two segments of double-stranded (ds)RNA that vary between 2827 (segment B) to 3261 (segment A) nucleotide base pairs. The larger segment A encodes a 110-kDa precursor protein in a single large open reading frame (polyprotein ORF) which is cleaved by autoproteolysis to form the mature viral proteins VP2, VP3 and VP4 (Hudson, P. J. et al., 1986). VP2 and VP3 are the major structural proteins of the virion. VP2 is the major host-protective immunogen of IBDV, and contains the antigenic regions responsible for the induction of neutralizing antibodies (Azad, et al., 1987). A second open reading frame (ORF), preceding and partially overlapping the polyprotein gene, encodes a non structural (NS) protein (VP5).
It has been demonstrated that the VP2 protein is the major host protective immunogen of IBDV, and that it contains the antigenic region responsible for the induction of neutralizing antibodies. This region containing the neutralization site has been shown to be highly conformation-dependent. The VP3 protein has been considered to be a group-specific antigen because it is recognized by monoclonal antibodies directed against it from strains of both serotype I and II viruses. The VP4 protein appears to be a virus-coded protease that is involved in the processing of a precursor polyprotein of the VP2, VP3 and VP4 proteins.
In the past, control of IBDV infection in young chickens has been achieved by live vaccination with avirulent strains, or principally by the transfer of maternal antibodies induced by the administration of live and killed IBDV vaccines to breeder hens. The strategy for the control of IBDV in chicks involves hyperimmunization of breeders, which allows them to transmit high levels of maternal antibodies to progeny during the critical first few weeks of life. Although maternal antibodies provide protection during this period, continued protection against IBDV must be maintained before the maternal immunity reaches sub-protective levels by the administration of live vaccines. However, maternal antibodies can neutralize a vaccine virus and reduce the viral load needed to induce immunity (Sharma et al., 1987). In addition, new antigenic variants of IBDV, which appeared during the 1980's, introduced new problems for poultry production. These new field isolates were able to break through neutralizing maternal antibodies induced by standard IBDV vaccines (Snyder, 1992). Since then, these variant strains have been incorporated into commercial inactivated vaccines for broiler breeders. For example, Mundt et al., 2003 vaccinated broilers, in ovo, with a chimeric IBDV vaccine using epitopes from classic and E/Del variant viruses, however, the challenge induced chronic lesions in BF of the vaccinated broilers.
Unfortunately, despite these vaccination measures, IBDV continues to be a problem. Very virulent strains of IBDV have caused outbreaks of disease with high mortality in Europe and Asia (Nunoya et al., 1992; van den Berg et al., 1991) despite vaccination programs. In addition, live vaccines that are available for mass vaccination of broilers in the first few weeks of life are not suitable for in ovo administration. These vaccines may induce immunosuppression during late stages of incubation, when the embryo is highly susceptible to infection.
Thus, it would be advantageous to develop a vaccine comprising a recombinant virus that can be administered in ovo but is not susceptible to the shortcomings of previous vaccines such as inducing immunosuppression during late stages of incubation or being neutralized by maternal antibodies.